Sole structure with r-f suppressors



Aug. 9, 1969 SUSUMU PAULOTSU-KA 3,462,637

SOLE STRUCTURE WITH R-F SUPPRESSQRS Filed Aug. 28, 1967 w I Lo 0 0 Yo 0 0 o o o I LAW 5 5 32 14g :1; f6 25 25 1 Z4 25 i i 4770i Mar United States Patent 3,462,637 SOLE STRUCTURE WITH R-F SUPPRESSORS Susumu Paul Otsuka, Palo Alto, Calif., assignor to Litton Precision Products, Inc., San Carlos, Calif., a corporation of Delaware Filed Aug. 28, 1967, Ser. No. 663,575 Int. Cl. H01j 25/34 U.S. Cl. 315-35 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to crossed field amplifiers, and more particularly, to a crossed field amplifier containing a sole electrode arrangement with electromagnetic energy suppression.

As is known, the crossed field amplifier is a microwave device which amplifies signals at high microwave frequencies. Conventionally, an interaction region is defined between a slow wave structure and a sole electrode. An electric field is established between the sole electrode and the slow wave structure, and a magnetic field is applied across the interaction region perpendicular to the direction of the electric field to establish a crossed field in the interaction region. A source of electrons located at one end of the interaction region provides electrons which are directed into the interaction region in a direction perpendicular to both the electric and magnetic fields. And a collector electrode is located at the other end of the interaction region to collect any electrons which have passed through the interaction region.

In such a device a signal, typically in the microwave frequency range, to be amplified is applied at an input to one end of the slow wave structure and propagates along the slow wave structure to an output terminal. Within the slow wave structure fundamental frequencies or spacial harmonics, here designed to be one of the frequencies to be amplified, having a predetermined phase velocity are created. And, according to well known electromagnetic theory, the stream of electrons directed into the interaction region at a velocity approximately equal to the phase velocity interacts with that electromagnetic energy to transfer energy thereto.

Initially, the electrons entering the interaction region must do so at the designed velocity determined by the ratio of E to B. Electrons faster or slower are initially sorted out by the well understood deflection principles for electrons traveling slower or faster than the ratio E/B. Since the electron travels close to the slow wave structure at a predetermined velocity, E/B, it interacts with the particular fundamental frequency or space harmonic on the slow wave structure having a phase velocity of approximately the same value. A worthwhile explanation of this phenomenon appears in U.S. Patent No. 3,325,677 granted to I. E. Orr.

In so interacting, the electron is slowed down slightly and accordingly falls closer to the slow wave structure. However, because of the magnetic field, any such particular electron is brought up to the correct velocity and continues interacting with the particular spacial harmonic. The electron in falling closer to the slow wave struture, however, undergoes a loss of potential energy. Ideally, this interaction continues for this and other electrons until they either lose all potential energy and fall into the slow Wave structure or are collected at the end of the interaction region. Thus, signals of the proper phase velocity applied to the input of the crossed field amplifier are amplified and appear as amplified signals at the output end of the slow wave structure.

While perfect in theory, in practice the cross field amplifier possesses some difiiculties. The cross field amplifiers for initially unexplainable reasons, outgassed. Normally, some loss material is provided within the amplifier to dissipate undesired electromagnetic energy. However, for unexplained reasons, the lossy material was being unduly heated to such an extent that it emitted gasses. Such gasses spoil the vacuum within the tube and cause the depositing of material on the tube elements that conse quently ruin the operation of the amplifier. Further increased loading and lower isolation between input and output were observed at particular frequencies within the operating frequency range of the amplifier. Speculation indicated that for some reason standing waves and oscillations were being created within the amplifier.

Since in a crossed field amplifier the sole electrode is normally positioned close to the slow wave structure, it appears that microwave energy could couple from the latter to the former. Where the inherent capacitance and inductances of the amplifier elements are such as to set up a resonance for such coupled energy, standing waves would be created on the sole electrode. That is, microwave frequency energy of an undesired frequency traveled back and forth along the surface of the sole electrode between the input and output terminals. With the creation of standing Waves, such microwave energy was capable of heating any lossy material present at an anti-node of such standing wave. The heating of such lossy material creates outgassing. Additionally, because of such spurious frequencies, the isolation between input and output terminals of the amplifier dropped considerably. And, with a lower isolation, the insertion loss of the output circuit increased. With increased insertion loss the available gain of the amplifier was lessened. Thus, as long as these conditions persist within the operating frequency range of the crossed field amplifier, the difficulties encountered would persist.

Conventionally, to rid an amplifier of spurious frequencies, lossy material is placed at selected locations within the tube so as to dissipate the undesired energy while not dissipating the electromagnetic Waves to be amplified. Because of the large bandwidth desired in an amplifier and the resultant unwieldy and unpredictable nature in selecting where the spurious frequency will appear, such a solution is not easily achieved. More often than is desired, the lossy material to an extent absorbs the energy to be amplified and complicates the manufacturing procedure because of the likelihood of outgassing.

Another solution to the problem was conjectured. That was to divide the sole electrode into a plurality of spaced segments so that it would ideally prevent spurious energy from traveling across the surface of the sole electrode.

However, because of the inherent capacitive coupling which exists between adjoining ends of such sole segments, this did not prove to be the solution and the problem remained. Because the difiiculties encountered were inherent in the nature of the construction of a crossed field amplifier, the unconventional nature of devices operating at microwave frequencies and the existence of these problems solely within the internal evacuated portion of the amplifier no simple solution existed.

Therefore, it is an object of the invention to eliminate one cause of outgassing within a crossed field amplifier.

It is another object of the invention to eliminate undesired resonances and oscillations within a crossed field amplifier.

It is a still further object of the invention to provide a crossed field amplifier having improved R-F isolation between the sole segments and the slow wave structure.

It is an additional object of the invention to prevent the setting up of standing waves at undesired frequencies within the crossed field amplifier.

Briefly stated, the invention encompasses the ordinary crossed field amplifier having the slow wave structure, electron source, collector electrode, and segmented sole electrode modified in that the segments of the sole electrode are connected together by small inductances, normally consisting of a few turns of wire in a coil. Such coil is of a proper value so as to cooperate with the inherent inductances -and capacitances formed by the tube elements which together are equivalent to a low pass filter with a cutoff freqency below that of the frequency range in which the crossed field amplifier is designed to operate.

The foregoing and other objects and advantages of the invention, together with its arrangement and form will be better understood 'by a consideration of the following detailed specification taken together with the figures of the drawing in which:

FIGURE 1 schematically illustrates a crossed field amplifier which embodies the invention;

FIGURE 2 schematically illustrates the relationship of several of the elements in the amplifier for purposes of analysis;

FIGURE 3 illustrates the equivalent inductances and impedances derived in an analysis of FIGURE 2;

FIGURE 4 illustrates an equivalent lumped parameter L-section low pass filter for the elements of FIG- URES 2 and 3.

And, FIGURE 5 illustrates a lumped parameter T- section low pass filter equivalent to the L-section filter of FIGURE 4.

In FIGURE 1 the amplifier 10, schematically illustrated, includes a suitable vacuum envelope, represented schematically by dashed lines 12, and magnetic pole pieces 14 for establishing a magnetic field of intensity B within the vacuum envelope. The magnetic field is represented by the encircled X in FIGURE 1. An electron gun consisting of a cathode 16, a grid 18, and an accelerator electrode is positioned at one end of amplifier 10. A sole electrode 24 which consists of a plurality of spaced segments 25 is provided. Each of sole segments 25 is supported by a ceramic insulator 26, such as aluminum oxide. Between each segment is a coil 28 consisting of a few turns of electrical conductor, such as Cupron. The ends of each coil 28 are connected between adjoining sole electrode segments 25. This places each of the sole segments at the same direct current electrical potential. Spaced from sole electrode 24 is a slow wave structure 30. The space between the slow wave structure 24 and sole electrode 30 define an interaction region 32 in which the electron beam from the electron gun interacts with an electromagnetic wave on slow wave structure 30.

While slow wave structure 30 is shown as a conventional interdigitated delay line, the invention is not limited to use with this particular delay line. Any suitable slow wave structure such as a helix may be substituted therefor without departing from the invention.

The electron collector region includes a conventional collector 34 positioned adjacent slow wave structure 30. An input 36 provides coupling for electromagnetic energy to the slow wave structure and an output 38 provides for removal of such energy from the slow wave structure.

As illustrated in FIGURE 1, slow wave structure 30 and collector 34 are maintained at ground potential. A variable voltage source V1 applies a relatively large negative voltage to cathode 16, voltage source V2 applies a negative voltage, relative to cathode 16 to sole electrode 24, voltage source V3 applies a negative voltage, relative to cathode 16, to grid 18, and voltage source V4 applies 4 a positive voltage, relative to cathode 16, to accelerator electrode 20.

While FIGURE 1 illustrates the arrangement in a linear device, the invention and the elements are equally applicable and equivalent to crossed field amplifiers which are circular in their geometry.

The details of construction of the crossed field amplifier are well known and understood by those skilled in the art. Hence, any further discussion of such conventional techniques does not add to the present specification.

The inductors 28 may be constructed of a material known as Cupron. The ends of these inductors are attached to individual sole segments 25 by conventional welding techniques. Additionally, the ceramic supports 26 may be constructed of aluminum oxide by beryllium oxide, as is conventional, and brazed in place both to the sole electrode and housing, as is also conventional.

The operation of the crossed field amplifier is well known and aptly described in the literature. Briefly, an electric field is established between the sole electrode 24 and the slow wave structure 30, represented by E in FIG- URE 1 and pole pieces 14 establish a magnetic field B in the direction indicated in FIGURE 1 perpendicular to the electric field. A signal of electromagnetic energy to be amplified is supplied to input 36 and propagates into and along the slow wave structure to the output terminal 38. An electron source, cathode 16, emits electrons which are deflected and accelerated by accelerator electrode 20 into the interaction region 32 between the slow wave structure 30 and the sole electrode 24 at a velocity of E/B. These electrons interact and transfer potential energy to the portion of the electromagnetic wave having a phase velocity, Vp, of a magnitude approximately equal to that of the velocity E/B of the electrons. Under these conditions there is a transfer of power or energy from the simple electron beam to an electromagnetic wave. Consequently, the electromagnetic wave applied to input 36 appears at the output 38 as an amplified signal.

When energy is taken from the electrons, they lose some potential energy. Those electrons which lose all potential energy by transferring it to the electromagnetic wave prior to leaving interaction region 32 fall into slow wave structure 20, whereas other electrons in the electron beam continue past interaction region 32 and are incident upon the collector electrode 34.

Ideally, the microwave frequency signals supplied to the input terminal of the crossed field amplifier remain solely on the slow wave structure. In practice, however, because the distance between the slow wave structure 30 and the sole electrode 24 is very small, the electromagnetic wave is there coupled across to the sole electrode will travel along sole electrode 24 to the other end where it subsequently couples back to the slow wave structure and the output. Because of the inherent capacitances and inductances in the crossed field amplifier, resonance conditions occurred at some frequencies which resulted in large reflection between input 36 and output 38 and the creation of a standing wave along sole electrode 24.

With the present invention illustrated in FIGURE 1, the coupling of microwave energy between the input of slow wave structure 30 and sole electrode 24 is substantially minimized or eliminated entirely. Any wave coupled to the sole electrode which would otherwise propagate along sole electrode 24 sees what is essentially a low pass filter having a cutofi frequency below that of the input frequency. Consequently, the sole electrode appears as a very high impedance to such signals. Thus, the microwave energy at input 36 sees the inductances coupled between sole segments in addition to the inherent inductance and capacitance of the sole electrode in the manner as is hereinafter analyzed and cannot propagate along the sole electrode.

Inasmuch as the ideal operation of the crossed field amplifier requires only a DC voltage the sole electrode for purposes of establishing the electric field E and does not encompass the appearance of any microwave frequency signals which would distort this field, the crossed field amplifier of the invention is more suited to the ideal type of operation.

FIGURE 2 represents a cross section of pertinent elements of the amplifier of FIGURE 1. This representation permits a quantitative analysis helpful in applying the invention to various configurations and in choosing proper dimensions. There is shown a segment of the slow wave structure 30, a sole electrode segment 25', a ceramic support 26' which supports the sole segment and a portion of the housing 12' upon which the ceramic support is placed.

The characteristic impedance and capacitance to ground of this configuration of elements may be looked upon, applied, and calculated using the seemingly unrelated strip transmission line theory. An elaboration of such theory appears in an article entitled, Strip Transmission Line by C. Bowness on pages 2 through 7 of the January 1956 issue of Electronic Engineering.

In fact, with great convenience, using the strip transmission line theory for flat strip lines above a ground plane permits one to analyze the inductance and capacitance separately for; one, the sole electrode 25 to the body structure 12; and two, the sole electrode 25' to slow wave structure 30'. By calculating each of these factors separately, the capacitance and inductance are then added in parallel to form a simple equivalent L filter section.

FIGURE 3 shows the equivalent capacitance C1 and inductance L1 between the sole segment 25' and the slow wave structure 30', and C2 and L2 illustrates the equivalent strip transmission components for sole segments 25 to housing 12'. According to strip transmission line theory, the capacitance C2 of the strip to ground plane is given by picofarads per meter length (pf.)/m. The characteristic impedance of the line is given by ohms. Therefore the series inductance of the line is given by picohenries per meter length ph/m. where e=th6 dielectrio constant of the insulator between the strip and ground plane, W=the width of the strip, and d=the distance between the strip and the ground plane.

Considering first the equivalent parameters between the sole segment and the slow wave structure in an exemplary crossed field amplifier, the distance to the strip to ground; that is, the distance between sole segment 25 and slow wave structure 30' in FIGURE 2 is approximately .20, and the width of the sole segment 25', W, is approximately 1.75". The dielectric between sole segment 25' and the slow wave structure 30 is vacuum and accordingly e=1.

Therefore, by substitution in these equations C =2.14 picofarads per inch (pf.)/inch and L1=3.350 picohenries per inch (ph.)/inch.

Next, considering the strip line equivalent parameters between sole electrode segment 25' and the housing or body 12' in FIGURE 2, the dimensions in an exemplary crossed field amplifier are such that the width, W, of sole segment 25 is approximately 1.75" and the distance of the segment 25' to housing 12' is approximately .50".

A ceramic support 26' is interposed between the sole segment and the housing, the dielectric constant, e, of which is equal to 9. However, because a portion of ceramic support 26 is narrower than and does not cover the entire sole segment 25', the analysis departs from the configuration of the ideal model in strip line transmission theory and requires an approximation for the average dielectric constant between sole segment 25' and housing 12' which varies according to its area relative to that of the sole segment. For convenience, e is chosen to be equal to 5. Thus, substitution in the aforecited equations yields the equivalent capacitance, C2 of FIG- URE 3, equal to 4.81 picofarads per inch of sole segment length and the equivalent inductance L2 equal to 7440 picohenries per inch of segment length in the exemplary amplifier having the cited dimensions.

To place the equivalent circuit of FIGURE 3 into the lumped parameter L-section of FIGURE 4, the following conventional filter circuit conversion is used: L L /(L L and C=C1+C2. To obtain the values of inductance and capacitance in the equivalent T-section, represented in FIGURE 5 each of the inductances determined for the capacitance of FIGURE 4 is halved or in other words, each inductance in FIGURE 5 equals half of the value of inductance of the inductor in FIGURE 4, while the capacitances are the same. To determine the overall lumped circuit equivalent circuits for the entire length of sole segment 25, the unit length values determined is simply multiplied by the total length of the sole electrode in inches. Insofar as the total length of the sole segment does not exceed 4% at the frequency of operation, this approximation results at most in 10% error.

Taking the L-section filter of FIGURE 4 to represent the equivalent inductance of the entire circuit and multiplying L by the exemplary 3" sole segment length, L in FIGURE 4 equals 6900 picohenries at the lumped capacitance C of FIGURE 4 equals 21 picofarads. As is apparent, this is a low pass filter and its cutoif frequency is determined by the formula fc=l/1r /LC which in the illustrated example is equal to 835 megacycles per second.

In the exemplary to crossed field amplifier under consideration, the frequency of 836 megacycles per second is in the range of frequencies supplied at an input for amplification. Thus, frequencies within the range of amplifier operation could circulate or couple between the slow wave structure and the sole electrode, and created a resonant condition with consequent standing waves, outgassing, and gain degeneration. Inasmuch as this low pass filter equivalent circuit characterization for the crossed field amplifier was developed, it became apparent that some means must be provided to lower the cutoff frequency of such low pass filter so that any coupling or current propagation along the sole electrode could be prevented. In the particular example, small coils 28 of conductive wire possessing an inductance of 0.2 microhenry was added between each of the adjoining sole segments 25 by simply connecting, by brazing or welding one end of coil 28 to one sole segment 25 and the other end of coil 28 to the adjoining sole segment. The inductance or inductive reactance which is provided by the addition of .2 microhenry between sole segments 25 is clearly represented in the T-section filter of FIGURE 5. Each of the two inductances illustrated in FIGURE 5 is increased by .1 microhenry.

In the L-section equivalent circuit low pass filter illustrated in FIGURE 4, the value of inductance L is increased by 0.2 microhenry. With such addition to either equivalent circuit, the cutoif frequency of the sole electrode becomes f=1/1r /(L+0.2 ,uh.)C which equals 153 megacycles per second. This cutoff frequency is below the frequency range of operation for the particular amplifier under consideration.

Thus, by modifying a conventional crossed field amplifier according to the teachings of the invention, RF or microwave energy coupling between input 36 and output 38 through the sole electrode 24 was substantially eliminated throughout the frequency band of amplifier operation. Accordingly, any resonance condition that could have occurred and did occur in prior art amplifiers which created standing waves and consequent outgassing of lossy material within the amplifier is substantially eliminated.

Another benefit achieved is that the insertion loss of the output disappeared within the operating frequency range of the amplifier, resulting in higher gain.

While the invention is thus disclosed in a particular embodiment and with particular exemplary dimensions and values are given to enable one skilled in the art to make and use the invention. It is expressly understood that the invention is not limited to the specific details and illustrations. As is apparent, many modifications suggest themselves to those skilled in the art and which lie within the spirit and scope of the invention.

Accordingly, it is intended that the invention be broad- 1y construed and limited in scope only by the appended claims.

What is claimed is:

1. In a microwave crossed field amplifier including a source of electrons, a slow Wave structure, a sole electrode spaced from said slow wave structure to form therebetween an interaction region, said sole electrode comprising a plurality of spaced segments, a collector electrode at one end of said interaction region for collecting electrons passing therethrough, means for propagating an electromagnetic wave along said slow wave structure, means for establishing an electric field in said interaction region between said sole electrode and said slow wave structure, and means for producing a magnetic field perpendicular to said electric field within said interaction region, and means for directing electrons into the other end of said interaction region at a predetermined velocity for interacting with an electromagnetic wave propagating along said slow wave structure; the improvement comprising: Inductive impedance means connected between adjoining sole electrode segments for lowering the cutoff frequency of said sole electrode.

2. The invention as defined in claim 1 wherein said inductive impedance means comprises an inductor.

3. The invention as defined in claim 2 wherein said inductor comprises a conductive wire coil.

References Cited UNITED STATES PATENTS 2,695,929 11/ 1954 Reverdin 3153.5 3,302,126 1/196-7 Orr 315-35 X 3,359,450 12/1967 Orr l -4 3153.5 3,385,994 5/1968 Hull 3l5-3.5

HERMAN KARL SAALBACH, Primary Examiner SAXFIELD CHATMON, JR., Assistant Examiner U.S. Cl. X.R. 

